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Re: Cladistic Idea

It seems to me that it would be interesting to approach a cladistic analysis of
fossil taxa one age at a time. As an oversimplified example, imagine an
analysis of ten fossil taxa, named "A" through "J". Imagine that
stratigraphically, they are arranged like this:

epoch #5: J
epoch #4: G,H,I
epoch #3: E,F
epoch #2: D
epoch #1: A, B, C

The standard way to do a cladistic analysis would be to take all ten taxa, score
them for all the characters, and away you go. My suggestion is to consider the
fauna of each age only in light of what has come before.

But the biota of epoch 1 didn't consist only of A, B and C. _At the minimum_, it consisted of A, B, C, and the ancestor of D through J!

By including D through J in the analysis, you effectively include that ancestor. By excluding them, your taxon sample is incomplete, and this can very easily result in a wrong tree topology. And if you include only some of them, you get a lopsided representation of that ancestor.

Naturally, it gets worse if the fossil record is worse and D through J isn't a clade that exclude A, B and C.

A bad fossil record also means you can't necessarily trust the stratigraphic ranges of the taxa you know. What if G, known only from epoch 4, actually existed from 2 to 5?